BoringSSL Style Guide

BoringSSL usually follows the Google C++ style guide, The rest of this document describes differences and clarifications on top of the base guide.

Legacy code

As a derivative of OpenSSL, BoringSSL contains a lot of legacy code that does not follow this style guide. Particularly where public API is concerned, balance consistency within a module with the benefits of a given rule. Module-wide deviations on naming should be respected while integer and return value conventions take precedence over consistency.

Modules from OpenSSL‘s legacy ASN.1 and X.509 stack are retained for compatibility and left largely unmodified. To ease importing patches from upstream, they match OpenSSL’s new indentation style. For Emacs, doc/openssl-c-indent.el from OpenSSL may be helpful in this.

Language

The majority of the project is in C, so C++-specific rules in the Google style guide do not apply. Support for C99 features depends on our target platforms. Typically, Chromium's target MSVC is the most restrictive.

Variable declarations in the middle of a function or inside a for loop are allowed and preferred where possible. Note that the common goto err cleanup pattern requires lifting some variable declarations.

Comments should be // C99-style for consistency with C++.

When declaring pointer types, * should be placed next to the variable name, not the type. So

uint8_t *ptr;

not

uint8_t* ptr;

Rather than malloc() and free(), use the wrappers OPENSSL_malloc() and OPENSSL_free(). Use the standard C assert() function freely.

Use the following wrappers, found in crypto/internal.h instead of the corresponding C standard library functions. They behave the same but avoid confusing undefined behavior.

OPENSSL_memchr

OPENSSL_memcmp

OPENSSL_memcpy

OPENSSL_memmove

OPENSSL_memset

For new constants, prefer enums when the values are sequential and typed constants for flags. If adding values to an existing set of #defines, continue with #define.

libssl

libssl was originally written in C but is being incrementally rewritten in C++11. As of writing, much of the style matches our C conventions rather than Google C++. Additionally, libssl on Linux currently may not depend on the C++ runtime. See the C++ utilities in ssl/internal.h for replacements for problematic C++ constructs. The util/check_imported_libraries.go script may be used with a shared library build to check if a new construct is okay.

If unsure, match surrounding code. Discrepancies between it and Google C++ style will be fixed over time.

Formatting

Single-statement blocks are not allowed. All conditions and loops must use braces:

if (foo) {
do_something();
}

not

if (foo)
do_something();

Integers

Prefer using explicitly-sized integers where appropriate rather than generic C ones. For instance, to represent a byte, use uint8_t, not unsigned char. Likewise, represent a two-byte field as uint16_t, not unsigned short.

Sizes are represented as size_t.

Within a struct that is retained across the lifetime of an SSL connection, if bounds of a size are known and it‘s easy, use a smaller integer type like uint8_t. This is a “free” connection footprint optimization for servers. Don’t make code significantly more complex for it, and do still check the bounds when passing in and out of the struct. This narrowing should not propagate to local variables and function parameters.

Naming

Follow Google naming conventions in C++ files. In C files, use the following naming conventions for consistency with existing OpenSSL and C styles:

Define structs with typedef named TYPE_NAME. The corresponding struct should be named struct type_name_st.

Name public functions as MODULE_function_name, unless the module already uses a different naming scheme for legacy reasons. The module name should be a type name if the function is a method of a particular type.

Some types are allocated within the library while others are initialized into a struct allocated by the caller, often on the stack. Name these functions TYPE_NAME_new/TYPE_NAME_free and TYPE_NAME_init/TYPE_NAME_cleanup, respectively. All TYPE_NAME_free functions must do nothing on NULL input.

If a variable is the length of a pointer value, it has the suffix _len. An output parameter is named out or has an out_ prefix. For instance, For instance:

uint8_t *out,
size_t *out_len,
const uint8_t *in,
size_t in_len,

Name public headers like include/openssl/evp.h with header guards like OPENSSL_HEADER_EVP_H. Name internal headers like crypto/ec/internal.h with header guards like OPENSSL_HEADER_EC_INTERNAL_H.

Return values

As even malloc may fail in BoringSSL, the vast majority of functions will have a failure case. Functions should return int with one on success and zero on error. Do not overload the return value to both signal success/failure and output an integer. For example:

OPENSSL_EXPORT int CBS_get_u16(CBS *cbs, uint16_t *out);

If a function needs more than a true/false result code, define an enum rather than arbitrarily assigning meaning to int values.

If a function outputs a pointer to an object on success and there are no other outputs, return the pointer directly and NULL on error.

Parameters

Where not constrained by legacy code, parameter order should be:

context parameters

output parameters

input parameters

For example,

/* CBB_add_asn sets |*out_contents| to a |CBB| into which the contents of an
* ASN.1 object can be written. The |tag| argument will be used as the tag for
* the object. It returns one on success or zero on error. */
OPENSSL_EXPORT int CBB_add_asn1(CBB *cbb, CBB *out_contents, unsigned tag);

Documentation

All public symbols must have a documentation comment in their header file. The style is based on that of Go. The first sentence begins with the symbol name, optionally prefixed with “A” or “An”. Apart from the initial mention of symbol, references to other symbols or parameter names should be surrounded by |pipes|.

Documentation should be concise but completely describe the exposed behavior of the function. Pay special note to success/failure behaviors and caller obligations on object lifetimes. If this sacrifices conciseness, consider simplifying the function's behavior.

// EVP_DigestVerifyUpdate appends |len| bytes from |data| to the data which
// will be verified by |EVP_DigestVerifyFinal|. It returns one on success and
// zero otherwise.
OPENSSL_EXPORT int EVP_DigestVerifyUpdate(EVP_MD_CTX *ctx, const void *data,
size_t len);

Explicitly mention any surprising edge cases or deviations from common return value patterns in legacy functions.

// RSA_private_encrypt encrypts |flen| bytes from |from| with the private key in
// |rsa| and writes the encrypted data to |to|. The |to| buffer must have at
// least |RSA_size| bytes of space. It returns the number of bytes written, or
// -1 on error. The |padding| argument must be one of the |RSA_*_PADDING|
// values. If in doubt, |RSA_PKCS1_PADDING| is the most common.
//
// WARNING: this function is dangerous because it breaks the usual return value
// convention. Use |RSA_sign_raw| instead.
OPENSSL_EXPORT int RSA_private_encrypt(int flen, const uint8_t *from,
uint8_t *to, RSA *rsa, int padding);

Document private functions in their internal.h header or, if static, where defined.

Build logic

BoringSSL is used by many projects with many different build tools. Reimplementing and maintaining build logic in each downstream build is cumbersome, so build logic should be avoided where possible. Platform-specific files should be excluded by wrapping the contents in #ifdefs, rather than computing platform-specific file lists. Generated source files such as perlasm and err_data.c may be used in the standalone CMake build but, for downstream builds, they should be pre-generated in generate_build_files.py.